emacs: show: increase default max-text-part-size

Currently notmuch-show-max-text-part-size is 10000 which means some
relatively normal messages have all parts hidden by default. Increase
this to 100000 by default.

The setting was introduced to alleviate problems with notmuch being
very slow on large threads. Users hitting these problems may wish to
customize this variable to something smaller (like 10000).